What it is
The shape is not decoration. A rounded, smooth-sided aluminium shell has far less wind resistance than a boxy caravan, which means a calmer ride, less sway and better fuel economy on long runs — which is why this shape has stayed in production since the 1930s.
Why people choose it
Durability is the other reason people buy one. A riveted aluminium skin does not rot the way wood-cored sidewalls do, so a well-maintained twenty-year-old unit can still be structurally sound.
What to check before you buy
The compromises: the interior layout rarely changes between models, repair parts are specific to the shape, and the rounded profile wastes floor area at the front and rear.